Soil respiration in six temperate forests in China

Scaling soil respiration (RS), the major CO₂ source to the atmosphere from terrestrial ecosystems, from chamber-based measurements to ecosystems requires studies on variations and correlations of RS from various biomes and across geographic regions. However, few studies on RS are available for Chinese temperate forest despite the importance of this forest in the national and global carbon budgets. In this study, we conducted 18-month RS measurements during 2004-2005 in six temperate forest types, representing the typical secondary forest ecosystems across various site conditions in northeastern China: Mongolian oak (Quercus mongolica Fisch.), aspen-birch (Populous davidiana Dode and Betula platyphylla Suk.), mixed deciduous (no dominant tree species), hardwood (dominated by Fraxinus mandshurica Rupr., Juglans mandshurica Maxim., and Phellodendron amurense Rupr.) forests, Korean pine (Pinus koraiensis Sieb. et Zucc.) and Dahurian larch (Larix gmelinii Rupr.) plantations. Our specific objectives were to: (1) explore relationships of RS against soil temperature and water content for the six forest ecosystems, (2) quantify annual soil surface CO₂ flux and its relations to belowground carbon storage, (3) examine seasonal variations in RS and related environmental factors, and (4) quantify among- and within-ecosystem variations in RS. The RS was positively correlated to soil temperature in all forest types, and was significantly influenced by the interactions of soil temperature and water content in the pine, larch, and mixed deciduous forests. The sensitivity of RS to soil temperature at 10 cm depth (Q₁₀) ranged from 2.61 in the oak forest to 3.75 in the aspen-birch forests. The Q₁₀ tended to increase with soil water content until reaching a threshold, and then decline. The annual RS for the larch, pine, hardwood, oak, mixed deciduous, and aspen-birch forests averaged 403, 514, 781, 785, 786, and 813 g C m⁻² yr⁻¹, respectively. The annual RS of the broadleaved forests was 72% greater than that of the coniferous forests. The annual RS was positively correlated to soil organic carbon (SOC) concentration at O horizon (R²=0.868) and total biomass of roots <0.5 cm in diameter (R²=0.748). The coefficient of variation (CV) of RS among forest types averaged 25% across the 18-month measurements. The CV of RS within plots varied from 20% to 27%, significantly (P<0.001) greater than those among plots (9-15%), indicating the importance of the fine-scaled heterogeneity in RS. This study emphasized that variations in soil respiration and potential sampling bias should be appropriately tackled for accurate soil CO₂ flux estimates.